1. Field of the Invention
This invention relates to a method of making a substantially ordered alloy by inter-diffusing the constituents of the alloy wherein at least one of the constituents is in the form of a thin film.
2. Description of the Prior Art
In normal substitutional solid solutions, the different kinds of atoms are arranged at random on a common lattice. There are some cases, however, where alloys which at high temperatures consist of random substitutional solid solutions undergo atomic rearrangement on slow cooling or annealing at a low temperature. This rearrangement produces an ordered structure in which the different kinds of atoms take up regular positions in the lattice. In copper-gold alloys, for example, an alloy containing 25 atomic percent gold (Cu.sub.3 Au) at high temperatures consists of a face-centered cubic structure with a random distribution of the two kinds of atoms. Upon annealing at a low temperature, the alloy is rearranged so that the gold atoms occupy the corners of the cube and the copper atoms occupy the centers of the faces. This type of structure is known as an ordered structure or sometimes as a superlattice.
When the atomic arrangement of alloys is transformed from the disordered phase to the ordered phase, various physical characteristics may be changed. Among the properties which may be changed as the alloy becomes more ordered are its electrical resistance, its Hall mobility, its magnetic properties, its Young's modulus, its volume, its mechanical hardness, its resistance to abrasion, its resistance to corrosion, and the like. Accordingly, ordered alloys can sometimes be used as magnetic recording media, in magnetic heads, for electrical contacts, ornaments and the like.
According to one prior art method of making a substantially ordered alloy, the metallic elements which form the desired alloy are melted with predetermined atomic percentages to prepare an alloy ingot which is then subjected to a suitable heat treatment. With this type of prior art method, it is difficult to make a desired ordered alloy, and it is virtually impossible to make such an alloy as a thin layer.
It has also been proposed in the prior art to make an ordered alloy by contacting metal plates made up of elements which will form the ordered alloy, and diffusing them by means of heat treatment. In such cases, the diffusion should be carried out at a temperature lower than the order-disorder transformation temperature of the ordered alloy, so that the diffusion requires a long period of time and is commercially impractical. If the diffusion is carried out at a temperature higher than the order-disorder transformation temperature, the concentration of the diffused element in one of the metal components is decreased from the surface to the interior exponentially and the diffusion portion which has the proper concentration ratio necessary to form the ordered alloy becomes extremely thin.